Many patents describe the use of ceramic materials in the filtration of particulates from fluids, such as the heated gases produced as a result of combustion processes. For the purposes of this invention, the term "fluid" includes a liquid, gas or flowable material, such as a molten metal. Such filters are necessary, for instance, to aid in the elimination of atmospheric pollution, the recovery of desirable particulates produced in high temperature processes, such as precious metals, and the enablement of energy savings through the recycling and reuse of hot purified gas in industrial processing. For instance, one process that produces such contaminated heated gases is the combustion of gasified coal. The exhaust gases from such combustion may reach temperatures in excess of 800.degree. C.-1000.degree. C. and contains undesirable levels of carbon-bearing particulates that should be filtered from the gases before the gases are released to the atmosphere.
Such filters may take the form of candle filters. Candle filters are self supporting and are shaped like long tubes, with one open end. Candle filters are typically fastened within an enclosure which is divided into "clean" and "dirty" sides such that the fluid to be filtered traverses from the dirty to the clean side of the enclosure by passing through the candle filter. The fluid flows typically from the outside to the inside of the candle filter, thus providing a fluid exiting from the open end thereof having an acceptable level of particulates.
Frequently, multiple candle filters are suspended from a common support structure within such an enclosure. The fluid containing the particulates is introduced into the enclosure below the common support structure and induced to flow through the walls of the candle filters and upwardly to the clean portion of the enclosure and outwardly through an exhaust port. Periodically, the filtered particulates accumulate on the exterior of the candle filters to a level that impedes the effective flow of the gases through the filters and the filtering of further particulates. A second pressurized fluid, typically air, is forcibly injected into the clean portion of the enclosure to flow in a reverse direction through the candle filters to dislodge the accumulation of particulates on the exteriors of the candle filters. The dislodged particulates fall to the bottom of the enclosure and may be removed, enabling normal operation of the filters to be resumed.
In gas filtration, much effort has been directed to making candle filters capable of withstanding high temperatures and chemically corrosive environments, such as is experienced with the combustion gases from coal gasification plants previously described. Ceramic materials have been found to be good candidates for candle filters under such conditions. However, the primary problem with ceramics is their brittleness. Hence, much of the conventional filter designs are directed to the manufacture of composite ceramic materials which, because of crack deflection and an increased relative flexibility, can attenuate or eliminate some of the traditional brittleness problems.
One example of a candle filter that has been developed in the past for such applications is found in U.S. Pat. No. 4,885,014, entitled "Apparatus For Filtering Gases". Filters such as that described in the '014 patent are marketed by Schumacher'sche Fabrik GmbH & Co. of Germany (hereinafter referred to as the "Schumacher" filter). The candle filter described in the '014 patent includes a truncated spherical cap at the open end of the filter. This spherical cap or "gimbal mount" is received within a perforated support plate which supports the candle filter within an enclosure and enables the filter to pivot with respect to the perforated support plate about a center that is aligned with the longitudinal axis of the filter and at the plane defined by the upper edge of the filter.
Pivoting of the filter with respect to the perforated plate may be induced by imbalances in fluid pressure about the filter, or by the accumulation of particulates filtered from the stream of gas by the filter. The accumulation of the filtered particulate matter may impinge on adjacent filters or the structure supporting the perforated plate. The lateral forces that these circumstances may apply to the candle filters may cause the candle filters to crack or break if the candle filter is fixed in place and not allowed to pivot.
The materials used to construct Schumacher filters consists of silicon carbide particulates bonded together by a clay matrix which becomes a glass upon firing. This structure results in thick walls making the filters relatively heavy. For instance, a Schumacher filter is typically 60 mm in external diameter and 30 mm in internal diameter. While this relative wall thickness assists in resisting cracking or breakage of the filter when subjected to lateral forces as described above, the resultant filter, which may reach lengths of 151.5 cm or more, may weigh 6.2 kg or more and thus becomes difficult to lift and manipulate manually. This makes the process of installing or removing and replacing such filters from common enclosures cumbersome and tiring.
Furthermore, because of the high density and thickness, Schumacher filters are subject to thermal shock, particularly during the pulse cleaning step. Typically, Schumacher filters have been fabricated from SiC particles with a glassy binder. The binder often contains additives to aid in forming the filters. This creates a potential problem of chemical reactions with constituents of some particulate bearing gases, such as sodium. For example, sodium is known to form a relatively low melting eutectic with silicon oxides at temperatures in the range of potential operation of the Schumacher filter; thus, creating an additional problem with commercial Schumacher filters of the prior art.